1. Field of the Invention
This invention relates to ferroelectric liquid crystal (FLC) devices, and particularly to a method and apparatus for driving the liquid crystal elements of such devices.
2. Description of Related Art
A ferroelectric liquid crystal has a permanent electric dipole which interacts with the applied electric field. Hence, ferroelectric liquid crystals exhibit fast response times, which make them suitable for use in display, switching and information processing applications. An example of an FLC device is described in a paper by N. A. Clarke et al, entitled "Submicrosecond bistable electro-optic switching in liquid crystals" in Appl. Phys. Lett. Volume 36, 1980, pp 899-901.
The stimulus to which an FLC device responds is a dc field, and its response is a function of the applied voltage (V) and the length of time (t) for which it is applied. The response is not a linear function of V.times.t, and there may be a voltage level at which, irrespective of the length of time for which the voltage is applied, switching of the device will not occur. There may also be a length of time of application of the voltage which will be too short for switching to occur, irrespective of the magnitude of the voltage.
An FLC device which can be miltiplexed needs to have at least two different states (called latched states) which the liquid crystal can adopt in the absence of an applied field. These can be the same states as the states (called switched states) obtained when a field of either polarity is applied, or they can be different states.
The liquid crystal can change from one switched state to another switched state when a field is applied thereto, without necessarily going to a latched state when the field is removed.
For a given time interval, the voltage at which the liquid crystal switches from one state to the other by 10% is called the switching threshold at 10% switching (T.sub.S10). The voltage at which the liquid crystal switches fully from one state to the other state is called the switching threshold at 100% switching (T.sub.S100). The voltage at which the liquid crystal will go fully into one of the latched states when the field is removed is called the latching threshold at 100% latching (T.sub.L100). The voltage at which the liquid crystal no longer goes into either of two different states when the field is removed is called the latching threshold at 0% latching (T.sub.Lo).
A ferroelectric liquid crystal element is switched to one state by the application of a voltage of a given polarity across its electrodes, and is switched to the other state by the application thereto of a voltage of the opposite polarity. It is essential that an overall dc voltage shall not be applied across such an element for an appreciable period, so that the elements remain charge-balanced, thereby avoiding decomposition of the crystal material. Pulsed operation of such elements has therefore been effected, with a pulse of one polarity being immediately followed by a pulse of the other polarity, so that there is no resultant dc polarisation.
The liquid crystal elements are commonly arranged in matrix formation and are operated selectively by energising relevant row and column lines. Time-division multiplexing is effected by applying pulses cyclically to the row (strobe) lines in sequence and by applying pulses, in synchronism therewith, to selected column (data) lines.
An example of an FLC display driving system is disclosed in an article by T. Harada, M. Taguchi, K. Iwasa and M. Kai in SID 85 Digest, p 131 et seq. This system uses four pulses per refresh cycle, and can therefore be classified as a 4 time slot system. For a 625-line display at video frame rates this would require a 16 .mu.s response of the crystal elements.